Near-field scanning optical microscopy (NSOM) is a type of microscopy where a sub-wavelength light source, usually a fiber tip with an aperture smaller than 100 nm, is used as a scanning probe over a sample. Near-field scanning optical microscopy is one in a family of scanned probe techniques that includes scanning tunneling microscopy and atomic force microscopy (AFM) where an image is obtained by raster scanning a probe across a surface collecting data at an array of points during the scan. In order to achieve an optical resolution better than the diffraction limit, the scanning probe has to be brought within the near-field region (that part of the radiated field nearest to the antenna, where the radiation pattern depends on the distance from the antenna). NSOM is based upon the detection of non-propagating evanescent waves in the near-field region. The probe is scanned over a surface of the sample at a height above the surface of a few nanometers and allows optical imaging with spatial resolution beyond the diffraction limit.
The scanning probe can either detect in the near-field directly, by means of the sub-wavelength size aperture (collection mode), or by using the probe as a waveguide with a sub-wavelength scattering source and detecting the evanescent waves as they propagate into the far-field (transmission mode). The achievable optical resolution of NSOM is mainly determined by the aperture size of the scanning probe and the probe-surface gap. NSOM may, in theory, be combined with any spectroscopic technique to gather spectra from small regions of a sample. Infrared (IR), Raman, visible, and V, as well as NSOM fluorescence, photoluminescence, photoconductance, and magnetooptical (MOKE) spectroscopies have been investigated.
Prior art techniques for nanoscale optical characterization imaging have been described in U.S. Pat. No. 5,489,774 entitled “Combined Atomic Force and Near Field Scanning Optical Microscope with Photosensitive Cantilever,” U.S. Pat. No. 6,985,223 entitled “Raman Imaging and Sensing Apparatus Employing Nanoantennas,” and U.S. Pat. No. 7,053,351 entitled “Near-Field Scanning Optical Microscope for Laser Machining of Micro- and Nano-Structures,” all of which are hereby incorporated by reference in their entireties for the teachings therein.
There is a need in the art for nanoscale optical probes that extend the measurements and standards infrastructure of conventional near-field scanning optical microscopy techniques.